It is well known that the standard vehicle having spaced apart drive wheels or wheel sets which are powered by a single engine through a differential drive experiences difficulties when one of the two differentially driven wheels loses traction. Conditions which give rise to a loss of traction exist commonly in construction sites and other off-road locations as well as on normal roads during wet, snowy or icy weather. A truck or automobile having one of two differentially driven wheels or wheel sets on ice and the other on ground providing good traction is often unable to move due to the fact that the action of the differential drive system directs all power to the wheel having no traction. The result is a slip condition wherein the wheel without traction rotates at twice its normal speed under given gear ratio specifications and the wheel with traction remains stationary.
To alleviate the slip or loss of traction condition, various mechanical anti-spin devices have been developed and put into commercial use. Such devices, however, can produce an abrupt transfer of all driving power to the wheel or wheel set having traction. This abrupt and full power transfer can create such mechanical stresses as to shorten the useful life of the drive train and/or cause catastrophic failure. In addition, mechanical anti-spin units often fail to accommodate the wheel speed differential which arises during normal turning of the vehicle and hence give rise to excessive tire wear due to drag effect.
An alternative approach to the slip problem due to loss of traction in differentially driven vehicles involves the provision of separately actuable drive wheel brakes whereby the operator can selectively apply a braking force to the spinning or slipping wheel thus to effect a balancing of power as between the slipping and nonslipping wheels; i.e., the application of the braking force to the slipping wheel simulates increased rolling resistance and results in a more even distribution of power as between the two differentially driven wheels. Systems of this type are common on farm vehicles but are not believed to be practical on large transport or off-road vehicles such as trucks and road graders.
More sophisticated approaches to slip control using the selectively actuable wheel brake systems are known in the prior art. These systems include speed sensors disposed on or adjacent each of the differentially driven wheels for generating speed signals, means for comparing the two signals to develop a slip signal and selectively operated solenoid means or solenoid operated valves to actuate either the left or right wheel brake when a slip condition is detected. One such system is disclosed in the U.S. Pat. No. 4,066,300 to Devlin issued Jan. 3, 1978. Another such system is disclosed in the U.S. Pat. No. 3,025,772 to Eger, Jr. et al. issued Mar. 20, 1962.
Neither the Devlin nor Eger, Jr. et al. system provides effective means for distinguishing between the signal resulting from a transducer failure and the signal resulting from a true slip condition wherein one wheel has lost traction. Both conditions include an indication that one wheel is stationary and the other is turning. Accordingly, automatic system actuation may occur when it is neither required nor advantageous to vehicle operation.
Another related prior art system is disclosed in the U.S. Pat. No. 3,871,249 to Jeffers, issued Mar. 18, 1975. In the Jeffers system, a mechanical locking device for a differential is equipped with a drive shaft speed sensor which inhibits lockup if shaft speed exceeds some predetermined amount. No means are provided for determining the relationship between shaft and wheel speeds. Hence Jeffers is not concerned with the problem of transducer failure and gives no teaching with respect to it.
The present invention is directed to overcoming the problems of the prior art and to provide an improved vehicle slip control system wherein slip is positively distinguished from transducer failure.